In order allow wireless communication channels to be set adjacently to each other as close as possible so as to effectively utilize frequency resources, a base station for mobile phones or the like requires a bandpass filter having a steep attenuation characteristic for preventing inter-channel interference. If a bandpass filter using a small-size and lightweight dielectric waveguide resonator, called a “dielectric waveguide filter”, is used in place of a large and heavy metal cavity resonator, the base station can be reduced in size and weight. It also becomes possible to facilitate a reduction in cost of the base station.
The dielectric waveguide filter is constructed by combining a plurality of dielectric waveguide resonators each having a dielectric block peripherally covered by a conductor film and partially provided with a coupling window through which the dielectric is exposed. Adjacent ones of the dielectric waveguide resonators are arranged in close contact relation, and a mutual coupling between the adjacent dielectric waveguide resonators is electromagnetically established through the coupling window. A coupling window having a long-side direction coincident with a direction of electric field is called an “inductive window”, and adapted to inductively couple adjacent dielectric waveguide resonators. A coupling window having a long-side direction perpendicular to a direction of electric field is called a “capacitive window”, and adapted to capacitively couple adjacent dielectric waveguide resonators.
Generally, to make an attenuation characteristic of a bandpass filter steep, the number of resonators constituting the filter may be increased.
However, an unloaded Q of a dielectric waveguide resonator is less than an unloaded Q of a metal cavity resonator. Thus, if the number of dielectric waveguide resonators in a dielectric waveguide filter is increased, an insertion loss in a passband of the filter will be increased. Therefore, a technique of forming attenuation poles by means of cross-coupling (bypass-coupling) is employed to obtain a filter having a low insertion loss and a steep attenuation characteristic, without increasing the number of dielectric waveguide resonators.
As a specific example of this conventional technique, a dielectric waveguide filter comprising four dielectric waveguide resonators and having attenuation poles formed by means of cross-coupling is disclosed in FIG. 5 of JP 2000-286606 A.
FIG. 8A is an exploded perspective view illustrating a conventional dielectric waveguide filter with attenuation poles using cross-coupling, and FIG. 8B is an equivalent circuit diagram corresponding to FIG. 8A. As illustrated in FIGS. 8A and 8B, the conventional dielectric waveguide filter 8 comprises six dielectric waveguide resonators 81 to 86 each having a rectangular parallelepiped-shaped dielectric block peripherally covered by a conductor film. The dielectric waveguide resonator 81 has an inductive window L81 for input, and the dielectric waveguide resonator 86 has an inductive window L87 for output. The dielectric waveguide resonators 81 to 86 are coupled in series through respective inductive windows L82 to L86, and a mutual coupling between the dielectric waveguide resonators 82, 85 is established through a capacitive window C80 in a cross (bypass)-coupling manner.
In this dielectric waveguide filter 8, a coupling path passing through the dielectric waveguide resonators 81, 82, 83, 84, 85, 86, and a coupling path passing through the dielectric waveguide resonators 81, 82, 85, 86, will hereinafter be referred to as “main coupling path” and “sub coupling path”, respectively.
In the dielectric waveguide filter, attenuation poles are formed by adjusting a transmission phase and a transmission amplitude in the sub coupling path, with respect to the main coupling path.
FIG. 9A is a graph illustrating a change in transmission phase to frequency in each of an inductive coupling path and a capacitive coupling path, wherein the solid line and the dashed line indicate a transmission phase in the inductive coupling path and a transmission phase in the capacitive coupling path, respectively. FIG. 9B is a graph illustrating a change in transmission phase to frequency in a dielectric waveguide resonator.
As illustrated in FIG. 9A, the transmission phase in each of the inductive coupling path and the capacitive coupling path is approximately constant irrespective of frequencies. The inductive coupling path has a function of advancing a signal phase by about 90 degree, and the capacitive coupling path has a function of delaying a signal phase by about 90 degrees.
On the other hand, as illustrated in FIG. 9B, the transmission phase in the dielectric waveguide resonator is delayed by 90 degrees on a low band side with respect to a resonant frequency f0 of the dielectric waveguide resonator, and advanced by 90 degrees on a high frequency side of a pass band (high band side) with respect to the resonant frequency f0.
Generally, in cases where a plurality of dielectric waveguide resonators are coupled in series, an inclination of the transmission phase becomes steeper as a path has a larger number of dielectric waveguide resonators.
Based on the above characteristics, a filter is designed such that a plurality of dielectric waveguide resonators are connected together while combining an inductive coupling path and a capacitive coupling path, and a signal transmitted through a main coupling path and a signal transmitted through a sub coupling path become opposite in phase and identical in amplitude.
For example, the dielectric waveguide filter 8 illustrated in FIG. 8A is designed such that a signal transmitted through the main coupling path and a signal transmitted through the sub coupling path become opposite in phase, on both of the low band and high band sides.
Such a design method is disclosed in J. Brain Thomas, “Cross-Coupling in Coaxial Cavity Filters-A Tutorial Overview”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 51, NO. 4, April 2003, P1368.
FIG. 10A is a graph illustrating respective transmission amplitude-frequency characteristic in the main and sub coupling paths of the dielectric waveguide filter 8 illustrated in FIG. 8A, wherein the solid line and the dashed line represent the main coupling path and the sub coupling path, respectively. FIG. 10B is a graph illustrating a transmission amplitude-frequency characteristic of the dielectric waveguide filter 8, which is obtained by synthesizing respective transmission amplitudes illustrated in FIG. 8A and phases in the main and sub coupling paths. In FIGS. 10A and 10B, a center frequency of the dielectric waveguide filter 8 is the resonant frequency f0, and two attenuation poles fa, fb are formed at frequencies at which the transmission amplitudes in the main and sub coupling paths are coincident with each other.
In FIGS. 10A and 10B, a distance between the attenuation pole fb and the resonant frequency f0 is greater than a distance between the attenuation pole fa and the resonant frequency f0. This is caused by the following low-pass filter-like property of the capacitive coupling path: a transmission amplitude becomes smaller along with an increase in frequency.
FIG. 11 is a graph illustrating respective transmission amplitude-frequency characteristic in a capacitive coupling path and an inductive coupling path, wherein the solid line and the dashed line represent the inductive coupling path and the capacitive coupling path, respectively. As illustrated in FIG. 11, a transmission amplitude in the inductive coupling path gradually becomes larger along with an increase in frequency, and a transmission amplitude in the capacitive coupling path gradually becomes smaller along with an increase in frequency. This means that the inductive coupling path has a high-pass filter-like property, and the capacitive coupling path has a low-pass filter-like property.